Radio frequency transponders are used in variety of applications. A typical transponder system operates to collect an electromagnetic signal/waveform and transmit a response signal/waveform, being similar to the collected signal or not.
For example, the known in the art Van-Atta type repeater is disclosed in U.S. Pat. No. 2,908,002. The Van-Alta type repeater is a passive electromagnetic device for receiving an incident electromagnetic wave and transmitting the received wave back in the direction from whence it has been received. The device includes a linear array of no less than four antennas, and means providing electromagnetic paths of equal length between antennas disposed symmetrically with respect to a geometrical center of said linear array.
U.S. Pat. No. 3,736,592 discloses a method and apparatus for obtaining automatic, selective retrodirective beam formation from a circularly symmetric antenna array. This system may be employed in an active or passive manner and accomplishes selective retrodirectivity by manipulation of beam terminals of a multiple beam matrix which in turn controls a multimodal network. The combination of the two matrix networks provide N separate beams from the circular antenna array. Also, by providing gain networks, control of the reradiated beam pattern is possible. This system has the ability to identify the angle of incidence of any particular transmission, and is particularly suited for navigational beacon systems since the reradiated signal can provide bearing information in response to interrogation.
Another example of a radio frequency transponder/repeater is disclosed in U.S. Pat. No. 4,806,938. Here, an array of receiving antenna elements is coupled to an array of transmitting antenna elements to provide a directional antenna system and including additionally an internal radio frequency source which is also coupled to the array of transmitting antenna elements. With such arrangement, an internally generated signal provided by the radio frequency source may be transmitted using the transmitting array of the Van Atta System.